1. Field of the Invention
The present invention relates to an oil passage structure of an engine for supplying oil to a crankshaft of an engine.
2. Description of the Related Art
In a multi-cylinder engine of, for example, vehicle, a steady oil supply is always required for lubrication between a crankpin of a crankshaft and a big end (one end portion having large-diameter than the other end portion) of a connecting rod.
As an ordinary lubrication system, an oil supply system for supplying oil from a plurality of journal walls formed in a cylinder block to respective crankpins through an oil passage formed in the crankshaft has been known (for example, see Patent Document 1 (International Laid-open Publication No. 2004/038188)). In the crankshaft described in the Patent Document 1,an inlet of an oil passage is opened in an outer peripheral surface (sliding contact surface) of each crank journal supported by each journal wall, and an outlet of the oil passage is opened in an outer peripheral surface (sliding contact surface) of each crankpin.
In this Patent Document, there is also disclosed that the oil passage in the crankshaft communicates with an oil passage in the journal wall via an oil groove of a sliding bearing. When the oil is fed from the oil passage in the journal wall to the oil groove of the sliding bearing, the oil flows into the oil passage in the crankshaft from the opening on the inlet side formed in the crank journal. Then, the oil flows through the oil passage in the crankshaft and is supplied between a crankpin and the large-diameter end portion of the connecting rod from the opening on the outlet side.
Furthermore, as another conventional lubrication system, there has been known an oil supply system for supplying oil from one end of a crankshaft to an oil passage formed in a crankshaft and then supplying the oil to a crankpin (for example, see Patent Document 2 (Japanese Patent Laid-Open Publication No. 2006-299830)).
In the crankshaft described in the Patent Document 2, an inlet of an oil passage is opened in one end face of the crankshaft and an outlet of the oil passage is formed in an outer peripheral surface of each crankpin. In this lubrication system, the oil flows into the oil passage in the crankshaft from the opening on the one end face of the crankshaft and flows through the oil passage and is supplied between a crankpin and a large-diameter end portion of a connecting rod from the opening on the outlet side.
In the lubrication system described in the Patent Document 1, the oil is supplied via the sliding bearing, so that an oil supply to the oil passage in the crankshaft is indirect. In addition, when the crankshaft is rotated at a high speed, the oil does not easily flow into the oil passage in the crankshaft from the opening on the inlet side of a crank journal due to centrifugal force. For this reason, there is a possibility that a shortage of oil supply to the crankpin will be caused. With this point, in the lubrication system described in the Patent Document 2,the oil from an oil pump is supplied directly to the oil passage in the crankshaft without flowing through a sliding bearing, and the oil is fed from the one end face of the crankshaft, thus applying less effect of the centrifugal force on the oil.
However, in the lubrication system described in the Patent Document 2,the oil flows from the one end face of the crankshaft, which requires a long oil passage to the crankpin positioned on the other end side of the crankshaft. For this reason, in a condition when the temperature is low and the viscosity of the oil is high, a delay in oil supply is caused to thereby raise the possibility of causing a shortage of oil supply to the crankpin positioned in the deepest portion of the crankshaft. To solve this inconvenience, although it may be thought to increase an amount of oil supply, the increasing in the oil amount may lead to another problem of increasing a discharge capacity of an oil pump, hence increasing a mechanical loss.